A waveguide/microstrip line converter has been widely used to convert and transmit a high-frequency signal between a waveguide and a microstrip line. In particular, according to a structure of a conventional waveguide/microstrip line converter used in a high-frequency band such as a millimeter wave band, a slot is provided in a tube wall of the waveguide and electromagnetically coupled to the microstrip line (see, for example, Patent Document 1).
The waveguide/microstrip line converter as described in Patent Document 1 has a structure in which the slot is provided in a direction perpendicular to a tube axis in the tube wall of the waveguide which is perpendicular to an electric field, the microstrip line is provided along the tube wall so as to be orthogonal to the slot, and the slot and the microstrip line are electromagnetically coupled to each other.    Patent Document 1: JP 09-246816 A